Electromigration is the mass transport of ions along a metal conductor as a result of a large electron current density in the conductor and is generally an undesirable phenomenon. The typical mechanism for electromigration damage to metallization lines in integrated circuits (ICs) is believed to be a two-stage process. In the first stage, it has been proposed that mass transport of conductor atoms results in the thinning of the conductor in some areas (for example, void formation), and the thickening of the conductor in other areas (for example, hillock formation). In the second stage, either the voids grow large enough to interrupt the flow of current, causing an open cicuit, or the hillocks grow out toward a neighboring metallization line, causing a short circuit. The consequence, in either case, is failure of the IC. Although electromigration damage leading to IC failure may generally be avoided by adherence to design rules limiting the current density in metallization lines to a prescribed value, such design rules are in at least some cases inconsistent with the requirements of VLSI circuit design. That is, because VLSI circuits have very small metallization cross sections, large current densities leading to electromigration damage can occur even for the relatively small currents typical of the operation of a VLSI circuit. As a consequence, electromigration is of great concern in assessing the reliability of ICs, and particularly VLSI circuits.
Conventionally, the reliability of a metallization line in an IC is evaluated by, e.g., a Median Time to Failure (MTF) test. That is, a large number of identically prepared thin film metallization lines are stressed to failure at an elevated temperature and current density. The time taken for half of the lines in the sample to fail gives a reliable indication of the propensity of similar lines to fail by electromigration. However, MTF testing can take days, or even weeks, to complete. During the testing period, the processing and packaging of ICs typically continues. As a consequence, if testing should reveal a problem in a particular manufacturing line, the inventory already processed in that line may be lost.
Although accelerated tests such as TRACE (Temperature Ramp Resistance Analysis to Characterize Electromigration) are available, which subject the sample to relatively high stresses, such tests provide little insight into the long term failure properties of thin metal films such as those used for IC metallization.
In at least some cases, by contrast, reliability with respect to electromigration can be assessed relatively quickly and non-destructively by means of tests based on noise measurement. That is, strong correlations are known to exist between the level and character of noise in thin metal films and the reliability of the films. (See, for example, J. L. Vossen, Applied Physics Letters, Vol. 23, pp. 287-289 (1973), and D. M. Fleetwood and N. Giordano, Physical Review B, Vol. 31, pp. 1157-1159 (1985).) In particular, the occurrence of electromigration and stress voiding in thin aluminum films subjected to high current densities has been correlated with a noise power spectral density (commonly referred to as the "noise power spectrum") having a 1/f.sup.2 dependence, where f represents frequency. Suppose a noise voltage associated with the current flow through a metal film having a finite resistance is measured and let S.sub.1/f.spsb.2 denote that component of the noise spectrum that is due to 1/f.sup.2 noise. It has been shown that S.sub.1/f.spsb.2 depends upon temperatures through a term of the form e.sup.-E.spsb.2.sup./kT, where E.sub.2 is an activation energy, k is Boltzmann's constant, and T represents temperature. Similarly, the time required for half the samples in an MTF test to fail is typically described by a formula, known as Black's formula, which depends upon temperature through a term of the form e.sup.-E.sbsp.a.sup./kT, where E.sub.a is also an activation energy. Significantly, the respective activation energies of the noise power spectrum and the MTF have been shown to be approximately the same for pure aluminum films. This correspondence, and the potential usefulness of noise measurements to determine the reliability of metallization lines in ICs, are discussed, for example, in J. G. Cottle, T. M. Chen, and K. P. Rodell, IEEE International Reliability Physics Symposium, pp. 203-208, (1988). Further discussions of the potential usefulness of noise measurements are found in A. Peczalski, IEEE Computer Society Test Technology Committee Curriculum for Test Technology, pp. 37-40, (1983), and A. Diligenti, P. E. Bagnoli, B. Neri, S. Bea and L. Mantellassi, Solid State Electronics, Vol. 32, No. 1, pp. 11-16, (1989).
However, the use of conventional noise-measurement techniques for assessing electromigration in thin metal films has generally been limited to packaged ICs and test structures using low-noise bonded contacts, in order to avoid measurement error caused by contact noise and mechanical vibrations. Although these techniques can provide useful information on the nature of electromigration in thin metal films, they are not a significant aid in an integrated circuit manufacturing environment, since these techniques would test for conductor reliability only after the manufacture of at least one IC is complete, i.e., after the IC is packaged.
By contrast, measurements performed in the manufacturing environment by the direct application of electrical probes to wafers are typically prone to errors caused by background noise, including contact noise and mechanical vibrations. Some reduction of background noise potentially is afforded by incorporating the film to be measured as one of the resistances making up a Wheatstone bridge. Resistance measurements using such an arrangement are insensitive to fluctuations in the source voltage, although other contributions to the background noise may still be significant. A direct current, Wheatstone bridge method was described, for example, by B. Neri, et al., "Electromigration and Low-Frequency Resistance Fluctuations in Aluminum Thin-Film Interconnections, " IEEE Transactions on Electron Devices, ED-34 pp. 2317-2321 (1987). However, Neri reported the use of additional techniques for the exclusion of background noise, including placing the measurement system on an antivibrating bench and enclosing it in a shielded room. Such measures would be difficult to implement in the manufacturing environment.
A particularly useful Wheatstone bridge method is described by J. H. Scofield, "AC Method for Measuring Low-Frequency Resistance Fluctuation Spectra," Rev. Sci. Instrum., Vol. 58, pp. 985-993 (1987). The Scofield method makes use of a center-tapped four-point probe for contacting the sample. That is, the sample resistance is divided into two equal portions by the center tap of the probe. Thus, the sample comprises not one, but two arms of a Wheatstone bridge. As in the case of a conventional bridge, two or more resistors (which comprise at least one ballast resistor and at least one measuring resistor) are connected in series with the sample, and the voltage source, which applies a voltage signal having an ac component, is connected between the center tap and a point intermediate the two ballast resistors. The ac component of the bridge imbalance signal appearing across a portion of the sample is amplified and demodulated by a phase sensitive amplifier. Like other bridge techniques, this measurement technique is relatively insensitive to fluctuations in the source voltage. Moreover, because the measurement technique is an ac technique, the amplifier can be used near its optimum frequency, and the use of phase sensitive detection is very effective in excluding background noise.
Scofield discusses the use of this technique for measuring resistance fluctuations in metal films. Scofield also mentions that for certain purposes it may be useful to apply a source voltage having both ac and dc components. Scofield does not, however, discuss any application of this technique for investigating phenomena related to electromigration, and in particular, he does not suggest the use of the dc component for stimulating electromigration. Moreover, the metal film samples discussed by Scofield were wirebonded to a 24-pin IC package prior to the measurements. However, the steps involved in wirebonding introduce a significant time delay between preparation of the films and their measurement. Such delay may be undesirable in applications of the measurement technique for the diagnosis of processes on an IC production line.
Thus, practitioners in the field have not yet achieved a quick, accurate reliability test based on the noise-measurement technique that avoids measurement errors due to mechanical and contact noise and can therefore be implemented on an integrated circuit manufacturing line. If achieved, such a testing method could reduce manufacturing time and expense. This application discloses such a testing method.